Different things work for different people. There are tablets, various kinds. Some take a while to work and others make you a bit drowsy. I find they usually help me. There are also wrist bands you can get (Boots have them), they dont make you drowsy and you dont have to put them ages in advance, unlike some tablets. However if you get really bad, you can get an injection from the ship's medical centre. Fortunately i've never been that bad. What does help prevet/reduce the severity of it, is cabin location. Avoid the bows or stern, especially on the higher decks because if the ship encounters heavy seas, you will feel far more motion that in a midship position.